1. Field of the Invention
The present invention relates to a sheet positioning device which performs positioning of a sheet member placed on a placement surface or platen, a sheet stacker provided with the sheet positioning device, an image forming apparatus, and an image scanner.
2. Discussion of the Related Art
A sheet positioning device has been incorporated in an image forming apparatus such as a copier and a printer, an image reader such as a scanner, and an automatic document feeder (hereinafter “ADF”) which are configured to use a sheet-shaped member. The sheet positioning device serves to align the sheet member such as a recording sheet, an OHP film, or an original sheet at a predetermined position on a platen or placement surface of the sheet. For example, the image forming apparatus in which a sheet stacker such as a sheet feed cassette or a manual sheet feed tray to contain recording sheets is incorporated has been known. In addition, the scanner or the ADF which incorporates a sheet positioning device on an original platen as a sheet placement surface to place the original sheet thereon has been known.
In such a sheet positioning device, the position of the sheet member placed on the sheet platen is regulated by a regulation member, thereby positioning the sheet member. For example, an image forming apparatus disclosed in JP-H07-267474-A includes a sheet positioning device which includes two side fences as regulation members, configured to slidably move back and forth in the direction perpendicular to the sheet conveyance direction on the platen. Home positions of these two side fences are greatly spaced away from each other. When a recording sheet is placed on the platen of the sheet positioning device, two side fences move to the respective home positions by being driven by a driving motor. In this state, when a bundle of sheets is set on the platen, the driving motor drives in such a direction opposite the above case that the two side fences approach each other. Then, either of the side fences contacts the recording sheet positioned to one side in the conveyance perpendicular direction of the sheet bundle and pushes it to a center. Thus, while each of two side fences pushing the sheet positioned to one side toward the center, a distance of the two side fences comes substantially closer to a sheet width of the recording sheet. Accordingly, the plurality of sheets placed on the platen with carelessness can be aligned to the center of the platen.
With such a structure, a sheet detector to detect whether a recording sheet is placed or not on the platen is preferably provided to prevent positional alignment of the side fences from being performed in a case in which the recording sheet is not placed on the platen. As a sheet detector, JP-2007-297190-A discloses a technology to detect a change in the position of a sheet detection lever. Specifically, the sheet detection lever of the sheet detector is configured to swing about a pivot shaft in a predetermined pivot angle. And when the recording sheet is not placed on the platen, the sheet detection lever is engaged to stop at a predetermined pivotally stop position by a biasing force of a spring. Then, in the thus engaged state, a detection target part of the sheet detection lever is detected by a transmissive photosensor. According to this structure, the sheet detector detects that the recording sheet is not placed on the platen. When the user places a recording sheet on the platen, the recording sheet contacts a rib end of the sheet detection lever to cause the sheet detection lever to move from the pivot stop position. Then, the transmissive photosensor does not detect the detection target part of the sheet detection lever. According to this structure, the sheet detector detects that the recording sheet is placed on the platen.
The inventor of the present invention has developed a manual tray including a sheet detector and a sheet positioning device. FIG. 1 is an oblique perspective view of the manual tray under development seen from above. An arrow B in FIG. 1 shows a direction along a surface of the platen of a manual tray 60 and perpendicular to a recording sheet conveyance direction on the platen, that is, the conveyance perpendicular direction. A broken line L1 shows a center line in the conveyance perpendicular direction of the manual tray 60. On the platen surface, a first side fence 611 and a second side fence 612, both being capable of slidably moving in the conveyance perpendicular direction, are disposed. The first side fence 611 regulates one end position of a recording sheet placed on the platen in the conveyance perpendicular direction of the sheet. The second side fence 612 regulates another end position of a recording sheet placed on the platen in the conveyance perpendicular direction of the sheet. These two side fences slidably move to approach the center line L1 with each other or to retract from the center line L1 with each other. In the illustrated state in FIG. 1, two side fences both stop at positions most spaced away from the center line L1 in the movable range thereof. The most spaced-away positions from each other are home positions of these two side fences. Each of the two side fences moves from each home position toward the center line by a distance corresponding to a sheet size in a state in which the recording sheet is placed on the platen, so that the center of the recording sheet in the conveyance perpendicular direction can be adjusted to the position of the center line L1.
Four sheet detectors are disposed at a leading end portion of the manual tray 60. These sheet detectors each include a sheet detection lever 699. Each sheet detection lever 699 is disposed at a positioned at predetermined intervals in the conveyance perpendicular direction as illustrated in FIG. 1, and a lever end which contacts the recording sheet is configured to protrude from a case of the tray directly upward. When the user places a recording sheet on the platen as illustrated in FIG. 2, the recording sheet P contacts any of the four sheet detection levers 699 to cause the contacted sheet detection lever 699 to move inside the case. According to this operation, the sheet detector detects that the recording sheet P is placed on the platen. Because a plurality of sheet detection levers 699 is disposed along the conveyance perpendicular direction, even though the recording sheet P is placed at any position, the existence of the recording sheet P can be detected.
However, this manual tray 60 includes a following disadvantage. Specifically, in a state as illustrated in FIG. 2, two of the four sheet detection levers 699 are moved inside the case due to contacting the recording sheet P. If in this case two side fences 611 and 612 are slidably moved toward the center line L1, as illustrated in FIG. 3, the recording sheet P is caused to be stuck with a side of the sheet detection lever 699 not contacting the recording sheet P and protruding directly upward from the case. Thus, there is a concern that the recording sheet P be folded or the sheet detection lever 699 or any driving system be damaged.
The present invention has been considered to obviate such a disadvantage and provides an optimal sheet positioning device capable of preventing the sheet member fed and adjusted in the conveyance perpendicular direction from being trapped by any movable contacting member such as a sheet detection lever.